Hello brayton69,
It sounds like you're unable to add a payment method to your kid's iPad because the postal code isn't being accepted, correct? We'd like to help, or get you in the best hands for assistance.
Just to confirm, after you enter the payment method, do you see a message saying the payment method was declined? If so, the information and steps here may help: If your payment method is declined in the App Store or iTunes Store
If that doesn't help, we'd like to get you in touch with Apple Support for further assistance. You can reach that team here: Get Support
Take care!